Sludge filtering and condensing equipment is disclosed in Patent Publications 60-59002 and 61-57043. The prior art filtering and condensing equipment described therein has filter plates arranged in a sludge tank, wherein one end of a filtrate discharge pipe is connected to the filter plate, and the other end is drawn out of the sludge tank. When the sludge tank is filled with sludge, sludge particles accumulate on the filter plate due to gravitational forces or by vacuum pressure acting on the inside of the filtrate discharge pipe. The water contained in the sludge passes into the filter plate, and is drawn from the sludge tank through the filtrate discharge pipe.
In particular, Patent Publication No. 60-59002 discloses sludge filtering and condensing equipment capable of forming uniformly thick cakes, wherein permeable filter plates having filter cloths affixed on both sides of the plates, are immersed in a tank containing a noncondensed sludge solution, and a filtrate discharging means is provided on said filter plates.
Additionally, Patent Publication No. 61-57043 discloses filtering and condensing equipment, in which water absorbing filter plates are disposed in a filtering and condensing tank, wherein one end of a filtrate discharge pipe is connected to the filter plate, while the other end is connected to a siphon tube disposed externally to the filtering and condensing equipment. The filtrate is siphoned from the top of the filter plate thereby forming uniformly thick cakes on the filter plate.
After condensed sludge has formed on the filter plates, the noncondensed sludge is removed from the tank and the condensed sludge is separated from the filter plate by blowing compressed air into the filter plate through, for example, the filtrate discharge pipe. Then, the separated condensed sludge falls to bottom of the sludge tank, and is removed from the tank.
The prior art achieves a higher concentration of condensed sludge by increasing both the vacuum pressure and the filtering period. Vacuum pressure is raised by extending the length of the filtrate discharge pipe downward outside of the sludge tank. For example, a negative pressure of 200 mmHg can be obtained when the exterior portion of the filtrate discharge pipe is 2.7 m in length, while increasing the length to 9.5 m will obtain a negative pressure of 700 mmHg.
By way of example, filtering purified water sludge with a solid concentration of 4% for 40 minutes with a vacuum pressure of 200 mmHg results in a condensed sludge with a solid concentration of 10%. Increasing the filtering time to 100 minutes produces a concentration of 11.2%.
Filtering purified water sludge with a solid concentration of 4% for 40 minutes with a negative pressure set to 700 mmHg results in a condensed sludge with solid concentration of 13.6%. Increasing the filtering period to 100 minutes produces condensed sludge with a solid concentration of 14.5%.